The present invention relates to a magneto-resistive device, and a magnetic head, a head suspension assembly and a magnetic disk apparatus which use the magneto-resistive device.
With the trend to a larger capacity and a smaller size of hard disk drives (HDD), heads are required to have a higher sensitivity and a larger output. To meet these requirements, strenuous efforts have been made to improve the characteristics of GMR heads (Giant Magneto-Resistive Head) currently available on the market. On the other hand, intense development is under way for a tunnel magneto-resistive head (TMR head) which can be expected to have a resistance changing ratio twice or more higher than the GMR head.
Generally, the GMR head differs from the TMR head in the head structure due to a difference in a direction in which a sense current is fed. A head structure adapted to feed a sense current in parallel with a film surface, as in a general GMR head, is referred to as a CIP (Current In Plane) structure, while a head structure adapted to feed a sense current perpendicularly to a film surface, as in the TMR head, is referred to as a CPP (Current Perpendicular to Plane) structure. Since the CPP structure can use a magnetic shield itself as an electrode, it is essentially free from short-circuiting between the magnetic shield and a device (defective insulation) which is a serious problem in reducing a lead gap in the CIP structure. For this reason, the CPP structure is significantly advantageous in providing a higher recording density.
Other than the TMR head, also known as a head in CPP structure is, for example, a CPP-GMR head which has the CPP structure, though a spin valve film (including a specular type and dual spin valve type magnetic multilayer film) is used for a magneto-resistive device (see JP-A-2003-60262 corresponding to U.S. patent application Publication No. 2003/0039080, and the like). In the TMR head, the magneto-resistive layer generally includes an oxide layer as a tunnel barrier layer. The CCP-GMR head, on the other hand, may include an oxide layer, as a thin insulating layer formed between two layers to such an extent that the insulating layer does not completely electrically insulate between the two layers, in order to effectively reduce the area of a path for a sense current (see JP-A-2003-60262).
Any type of CPP-based heads has an upper electrode and a lower electrode for supplying a current to a magneto-resistive layer formed on a base, formed on the top (opposite to the base) and on the bottom (close to the base) of the magneto-resistive layer, respectively,. Generally, for reasons of manufacturing process, the base formed with the magneto-resistive layer is left in the atmosphere after the magneto-resistive layer is formed and before the upper electrode is formed. In this event, for preventing the top surface of the magneto-resistive layer from being oxidized in the air to damage the characteristics of the magneto-resistive layer such as an MR ratio, a non-magnetic metal layer, referred to as a “cap layer”, is previously formed as a protection film on the top surface of the magneto-resistive layer (see JP-A-2002-216321 corresponding to U.S. patent application Publication No. 2002/0135951, JP-A-2000-228003 corresponding to U.S. Pat. No. 6,452,385, JP-A-2003-60262 corresponding to U.S. patent application Publication No. 2003/0039080, and the like). JP-A-2002-216321 discloses a cap layer comprised of a single-layer film made of Ta, Cr, NiCr, Ti or TiW, while JP-A-2000-228003 discloses a cap layer comprised of a single-layer film made of Cu, Al, Rh, Ru, Pt, RuRhMn, PtMn, PtMnRh, or TiW, respectively. JP-A-2000-228003 also discloses a cap layer comprised of a two-layer film made up of a Ta layer (upper) and a PtMn layer (lower (closer to the magneto-resistive layer)); a two-layer film made up of a Ta layer (upper) and a Cu layer (lower); a two-layer film made up of a Ta layer (upper) and an Al layer (lower); a two-layer film made up of a Ta layer (upper) and a Ru layer (lower); a two-layer film made up of a TiW layer (upper) and a Cu layer (lower); a two-layer film made up of a TiW layer (upper) and a Rh layer (lower); or a two-layer film made up of a TiW layer (upper) and a Ru layer (lower). Further, JP-A-2003-60262 discloses a cap layer comprised of a single-layer film or a composite-layer film made of simple Ru, Rh, Os, W, Pd, Pt, Cu, or Ag, or an alloy made up of two or more of these elements in combination. In the CPP-based head, the upper electrode is electrically connected to the magneto-resistive layer through the cap layer.
As will be understood from the foregoing discussion, in the CPP-based magnetic head, for reasons of manufacturing process, the base formed with the magneto-resistive layer and the cap layer is once placed in the atmosphere after the formation of the magneto-resistive layer and cap layer and before the formation of the upper electrode. Subsequently, the upper electrode is formed through several steps, wherein the head characteristic (MR ratio) will be degraded unless a contact resistance is minimized between the cap layer and the upper electrode. The contact resistance is caused by an oxide film on the surface of the cap layer which is formed when the base is exposed to the atmosphere. To address this problem, the surface oxide film on the cap layer is removed by dry etching (including a whole dry process such as sputter etching, ion beam etching, and the like) within the same vacuum chamber in which the upper electrode and the like are deposited, prior to the formation of another layer such as the upper electrode on the cap layer. In other words, the surface of the cap layer is cleaned to reduce the contact resistance.
A thick oxide film can be formed depending on the material for the cap layer. In this event, for sufficiently removing the thick oxide film by dry etching, a long cleaning time is required, resulting in a failure in efficiently manufacturing magnetic heads and an inevitable increase in cost. On the other hand, when a thinner cap layer is formed with the intention of reducing the gap between an upper magnetic shield and a lower magnetic shield to increase the magnetic recording density, long-time ion beam etching or the like performed to sufficiently remove the thick oxide film would increase damages to the magneto-resistive layer (particularly, a tunnel barrier layer or the like) due to the ion beam, resulting in a lower MR ratio.
Therefore, in the CPP-based head, the material for the cap layer is selected from a viewpoint of insusceptibility to oxidization, or a lower resistance even in the event of oxidization, and the cap layer is generally comprised of a single-layer film made of this material. Specifically, the cap layer is generally comprised, for example, of a single-layer film made of Ta or the like, or a single-layer film made of a noble metal such as Rh, Ru, or the like.
As described above, JP-A-2000-228003 also discloses a cap layer comprised of a two-layer film such as a two-layer film made up of a Ta layer (upper) and a Ru layer (lower (closer to the magneto-resistive layer)), and the like. The noble metal layer such as a Ru layer is formed in order to reduce the degree of diffusion of a constituent element in the cap layer (particularly, a constituent element in the upper Ta layer and the like) into the magneto-resistive layer (particularly, the free layer). Therefore, in JP-A-2000-228003, a noble metal layer such as a Ru layer, which forms part of a cap layer, is located closer to a magneto-resistive layer. Therefore, JP-A-2000-228003 does not disclose or suggest the opposite order of the lower noble metal layer and the upper Ta layer. In addition, while JP-A-2003-60262 also discloses a cap layer comprised of a composite-layer film as mentioned above, these materials are selected merely from a viewpoint of insusceptibility to oxidization, or a lower resistance even in the event of oxidization.
Magnetic heads are desired to provide a higher MR ratio for a higher recording density and the like. Also, as will be understood from the foregoing discussion, a reduction in the time required for cleaning the surface of the cap layer could permit magnetic heads to be efficiently manufactured, and would reduce the likelihood of a lower MR ratio, even with a thinner cap layer, because the magneto-resistive layer would be less damaged by an ion beam. These aspects are similarly applied not only to magneto-resistive devices employed in magnetic heads, but also to magneto-resistive devices employed in other applications.